From: Lyle Johnson <ljohnson@...>
Date: 2001-08-08T13:35:29+09:00
Subject: [ruby-talk:19344] Re: Virtual list

> Ah, see, the issue is that the Ruby objects won't exist to put into an
> array.  I need kind of a "virtual" array that will create the Ruby
> objects on the fly from the C++ objects in the C++ list, when the Ruby
> array is accessed.  Also, when a new Ruby object is placed into the
> array, the C++ class needs to be pulled out (check the Ruby class of the
> object, of course) which would mean something like
>
> obj.attrs.add (AMAttribute.new ("type", "data", "etc"))
>
> would create an instance of the C++ class obj_attr (wrapped in the class
> AMAttribute in Ruby), and pass that to the attrs object, which would
> basically take the obj_attr pointer from the AMAttribute instance, and
> call the following C++ code:
>
> obj->attrs.add (obj_attr_instance);
>
> The each member function of the attrs class (in Ruby) would call C++
> code that would loop thru all the elements in the C++ list, construct
> the Ruby wrapper for the class, and call the yield'ed function on them.

It sounds like your C++ extension module needs to define at least the
following basic classes:

    AMObject
    AMAttribute
    AMContainer

as well as (maybe) these list classes:

    class AMAttributeList {
    protected:
        std::list<obj_attr *> _attributes;
    public:
        AMAttributeList(std::list<obj_attr *>& attributes) :
_attributes(attributes) {}
        void add(obj_attr *attribute) {
            _attributes.add(attribute);
        }
        // and whatever else is required
    };

    class AMContainerList {
    protected:
        std::list<container *> _containers;
    public:
        AMContainerList(std::list<container *>& containers) :
_containers(containers) {}
        void add(container *c) {
            _containers.add(c);
        }
        // and whatever else is required
    };

When the Ruby code calls your AMObject's "attrs" accessor method, it should
return a reference to a wrapped AMAttributeList; when Ruby code calls your
"containers" method (or whatever you decide to call it), AMObject should
return a reference to a wrapped AMContainerList.

Now, as for the interfaces for these two new list classes, you should make
them as similar to Ruby's built-in arrays as you can ;) At a minimum, it
looks like you want to include an "each" method and mix in the "Enumerable"
module. Normal Ruby arrays also provide indexed access using the "[]" and
"[]=" methods, but if your underlying STL container is std::list<> this may
not be an option (at least, not an efficient option).

Hope this helps,

Lyle